Generally, in an lithography process for manufacturing microdevices such as a semiconductor device, a display device (such as a liquid crystal display device), an imaging device (such as a CCD), a thin-film magnetic head, or a micromachine, various exposure apparatus are used that transfer a pattern formed on a mask or reticle (hereinafter generally referred to as ‘reticle’) onto an object such as a wafer or a glass plate (hereinafter generally referred to as ‘wafer’). In recent years, from the viewpoint of placing importance on throughput, a projection exposure apparatus of a sequentially moving type is mainly used, such as a reduction projection exposure apparatus (the so-called stepper) by a step-and-repeat method, or a scanning exposure apparatus (the so-called scanner (also referred to as a scanning stepper) by a step-and-scan method.
Because this type of projection exposure apparatus is used for manufacturing microdevices, in order to make a device, which serves as an end product, exert a desired performance, it is important to accurately form a reduced image (an image reduced to a size according to a projection magnification that has a similar shape to an original pattern) of a pattern formed on a reticle that corresponds to a projection magnification of a projection optical system on a wafer. Especially in a transferred image of a pattern that has the same size as the pattern on the reticle, it is important that the transferred image can be formed in the same size within the entire effective field or exposure field of the projection optical system. That is, the in-plane uniformity of the pattern size is important. For example, in the case of a line pattern, the line width of the image should be uniform within the plane, while the line width uniformity is important also in between vertical lines and horizontal lines.
The in-plane uniformity of the pattern size described above or the like is greatly influenced by the image-forming characteristics of the projection optical system, and for example, in the case aberrations such as curvature of field, spherical aberration, coma, or distortion exist in the projection optical system, the formed state of the image of the pattern of the same size formed at different positions will be different. Further, in the case astigmatism exists in the projection optical system, the formed state of a resist image of a horizontal line pattern and a resist image of a vertical line pattern of the same size will be different. This means, when expressed from an opposite point of view, that in the case a plurality of transferred images (e.g. resist images) of a pattern of the same size each transferred on a wafer via the projection optical system is measured, the image-forming characteristics of the projection optical system can be determined based on the measurement results, and furthermore, adjustment of the exposure apparatus such as the adjustment of the image-forming characteristics of the projection optical system becomes possible.
In recent years, due to finer patterns, when measuring the size of the resist image formed on the wafer (e.g. the line width or the like in the case of a line pattern), a scanning electron microscope (SEM) (hereinafter simply referred to as ‘SEM’) for length measuring is normally used as the measurement unit.
However, in the case a pattern on a reticle is transferred onto a wafer by an exposure apparatus, and line width measurement of a resist image formed on substantially the same position on the wafer is performed using a commercially available SEM system for length measurement after the wafer has been developed, the situation has frequently begun to occur where the measurement results contain a line width difference in the resist image of a vertical line pattern and the resist image of a horizontal line pattern that cannot satisfy the specification of pattern line width controllability required in recent exposure apparatus even if adjustment of the exposure apparatus has been repeatedly performed countless times, which unexpectedly increases the start-up time of the exposure apparatus within the semiconductor factory.
Because it is certain that the specification of the pattern line width controllability required in the exposure apparatus will become tighter in the future, it is important to develop a new technology that can reduce the line width difference described above.